T 2 relaxation of the parotid gland of patients affected by pleomorphic adenoma

Abstract

The T 2 behavior of parotid gland tissue was investigated in 11 patients affected by pleomorphic adenoma. A protocol that was previously set up to define acquisition and post-processing procedures, reaching an accuracy of 2.5% in phantoms and an in vivo long term reproducibility of 0.9–8.5%, was used for the evaluations. The measurements were carried out on a whole body, superconducting imager, using a neck coil as a receiver. Some reference gel samples were imaged together with the patient and used to correct T 2 results. The sequence protocol was a multispin-echo, 16 echoes. Signals were fitted with mono and biexponential decay models and an automatic choice of the best model was performed using the two chisquared comparison. Two T 2 maps (T 2 monoexponential or short T 2 component, and long T 2 component) and chisquared maps were then produced. Pathologic and normal tissues showed a dominant monoexponential decay with a good level of biexponentiality (16%–27% of total fitted pixels) due to partial volume effects from the liquid content. Concerning the biexponentiality, no significant differences were found between the fitted pixel fraction of normal and pathologic tissue, because the T 2 long component of the lesion was related both to the edema and saliva content, but probably the increase in the first compensated the decrease in the second. Chisquared maps showed that most of the lesions presented a monoexponential core surrounded by a biexponential border probably due to a solid component similar to normal tissue with partial volume effects from saliva content. Ninety-five percent confidence intervals for normal tissue were 69.40–87.80 ms (monoexponential relaxation), 38.19–44.67 ms and 285.84–691.28 ms (short and long components of biexponential relaxation). For pathologic tissue they resulted 172.17–275.83 ms, 53.86–89.98 ms and 442.10–814.58 ms. The monoexponential component, mostly present in the core of the lesion, was the parameter that better characterized pathologic tissue. A comparison was performed between normal tissue of patients and normal tissue of volunteers, whose statistics was collected in a previous study with the same evaluation protocol. Results showed no significant differences in the biexponential fitted fractions and the comparison of relaxation times. We conclude that, for tissue characterization, a multiexponential analysis should be carried out in order to improve accuracy and to obtain more reliable results. Moreover, other than relaxation calculations, a topographical analysis of relaxation distribution, using for instance the chisquared maps, might in the future give us more useful information on tissue structure.